Inks used in non-impact printing, including thermal ink-jet printing, bubble jet printing, piezoelectric printing, and others (collectively referred to herein at ink-jet), typically comprise a colorant and a vehicle, with the vehicle often containing water and other relatively low surface tension liquids. The colorant is typically a dye or pigment.
Pigment-based ink offers a variety of desirable properties over dye-based ink, viz., waterfastness, lightfastness, durability, etc. However, preparation of pigment-based ink-jet inks has not been a widely commercialized due to stability problems of the water insoluble pigments, the tendency of pigments to agglomerate, precipitation or settling of the colorants, and very short dispersion life. Another problem associated with pigment-based inks is the tendency to clog components of the printers such as the nozzles.
The tight tolerances of the nozzles (typically 50 .mu.m diameter) require that the ink not contain relatively large particles which tend to clog the nozzles, leading to a significant negative impact on the performance of carbon black pigmented inks. Studies have shown that nozzle problems are a potential problem with inks that contain even a very low level of particles ranging is size from about 0.5 to about 2 .mu.m.
It is believed that even part-per-million (ppm) levels of oversize pigment particles in the ink settle out in the nozzle area. Firing the pen then converts this sediment into a hard mass that cannot be dislodged with servicing or wiping. As stated above, evidence suggests that the size of these particles is on the order of 0.5 .mu.m to 2 .mu.m. In theory, a 0.3 micron filtration process should remove these particles. However, the micron rating of filter media can be misleading; filters are typically around 98% efficient, so low concentrations of particles larger than 0.3 .mu.m do slip through and eventually settle out in the nozzle area. Traditionally, industry has relied upon various techniques, such as filtration, microfluidizers, and high shear mixers, to lessen the problem; unfortunately, the results of using these techniques in manufacturing has produced marginal or moderate degrees of success.
Moreover, the amount of larger particles at the ppm level is very difficult to measure using conventional particle sizing techniques, such as microtrec or nicomp instruments. Therefore, it would be advantageous to have an easy method to measure the level of larger particles present and relate this information to ink performance.
Accordingly, investigations continue into developing ink formulations utilizing pigments which have overcome the problems associated with agglomeration, clogging of nozzles and the presence of large particles, and also provide a reliable method to verify that these particles are not present in an amount sufficient to cause these problems.